1. Field of the Invention
The invention relates to a method and system for multiplexing or demultiplexing transmission channels of a communication network, e.g. a third generation cellular network.
2. Description of the Related Art
Within the International Telecommunications Union (ITU), several different air interfaces are defined for third generation mobile communication systems, based on either Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) technology. Wideband CDMA (WCDMA) is the main third generation air interface and will be deployed in Europe and Asia, including Japan and Korea, in the same frequency band, around 2 GHz.
WCDMA technology has shaped the WCDMA radio access network architecture due to the requirements of CDMA basic features, such as soft handover which is a category of handover procedures where the radio links are added and abandoned in such a manner that the terminal device, or user equipment (UE) in third generation terms, keeps at least one radio link to the radio access network.
The WCDMA air interface has been defined to provide, in the first phase, data rates up to 2 Mbps in the 3GPP (third generation partnership project) Release 99 and Release 4 specifications. In the Release 5 specification, peak data rates up to 10 Mbps are possible with a high speed downlink packet access (HSDPA) feature to thereby support packet-based multimedia services. In HSDPA, the intelligence of the Node B, which is the third generation equivalent to the former base station, is increased for handling of retransmissions and scheduling functions, thus reducing the roundtrip delay between a mobile device and the network entity handling retransmissions, e.g. the radio network controller (RNC). This makes retransmission combining feasible in the mobile device due to reduced memory requirements. In general, all HSDPA users share the channel in both time and code domains. Adaptive modulation and coding are used to support multiple rate transmissions for different types of multimedia services.
A low client-server round-trip time (RTT) is of great importance for applications based on the Transmission Control Protocol (TCP), where the congestion control mechanisms of TCP limit the data rate as a function of the observed RTT. An RTT which is too large may, in this case, lead to inefficiency in the radio-resource utilization and degraded end-user performance.
In WCDMA systems, the transmission time interval (TTI) is defined as the inter-arrival time of transport block sets, i.e. the time it takes to transmit a transport block set. The transport block set is defined as a set of protocol data units (PDUs) exchanged between the physical layer (L1) and the Medium Access Control (MAC) which is a sublayer of the radio interface layer 2 (L2) providing unacknowledged data transfer service on logical channels and access to transport channels. Shortening the uplink TTI will contribute to an overall client-server RTT reduction. In addition to the obvious reduction in delay from the TTI itself, a reduced TTI allows for reduced processing delays as well. Incoming data to be transmitted need to wait until the start of the next TTI, a waiting time which is shortened with a reduced TTI. Furthermore, the smaller payload resulting from a reduced TTI (assuming unchanged data rate) allows for a reduced processing time in the decoding process. A shorter uplink TTI should allow for a significant uplink-delay reduction while still supporting reasonable payloads.
In the following, the abbreviation “E-DCH” is used to denote a new transport channel type, supporting a shorter TTI of 2 ms. Enhanced uplink DCH (E-DCH) is being studied in 3GPP (Third Generation Partnership Project). The targets are increased cell and user throughput and shorter delay. Possible enhancements studied are fast Node B based scheduling, fast (H)ARQ ((Hybrid) Automatic Repeat Request) between UE and Node B and shorter TTI length, i.e. less than 10 ms. One motivation in the E-DCH to which TTI length is strongly related is to minimize the air interface delay. The selection between the TTI lengths will also depend on which TTI length the multiplexing scheme is possible to design at reasonable increase in complexity, what kind of peak to average power ratio (PAR) it will result in, and what kind of performance it will result, etc. Also the TTI length should be selected in such way that several services can be served simultaneously on E-DCH and DCH, since having several simultaneous services is an essential feature of UTRAN (Universal Mobile Telecommunications System Terrestrial Radio Access Network).
Shorter TTI is easily introduced by having it on a separate code channel, i.e., by code multiplexing it. This, however, increases the PAR in the UE transmitter, which requires more linear power amplifier and makes the power amplifier less efficient. Therefore, it is desirable to time multiplex the new E-DCH(s) having shorter than 10 ms TTI, e.g. 2 ms or 3.3 ms (i.e., 3 or 5 slots, respectively), with the normal DCHs having TTI length of 10 ms or larger, e.g. 10, 20, 40 and 80 ms are currently allowed in the WCDMA specifications. So the problem is, how to time multiplex E-DCH having shorter than 10 ms TTI with DCHs having 10 ms or larger TTI. Furthermore, the solution should be such that legacy Node Bs are still able to decode the normal DCHs in a soft handover (SHO) situation where one or more of the active set Node Bs are legacy Node Bs.
Time-multiplexing proposals have been made e.g. by TSG RAN WG1 Tdoc R1-03-0211 or TSG-RAN WG1 Tdoc R1-03-0274. In both proposals, the time multiplexing of 2 ms TTI with 10 ms TTI was concluded to be relatively complex. In TSG RAN WG1 Tdoc R1-03-0211 it has been proposed to separate 10 ms and 2 ms TTI into different radio frames (10 ms), i.e., in one 10 ms radio frame either 2 ms TTI is used (i.e., 5 TTIs) or multiples of 10 ms TTI. So the time multiplexing is at radio frame level. It was assumed that there is a fixed switching point between 2 ms TTI and 10 ms TTI, which made the time multiplexing complex. In TSG-RAN WG1 Tdoc R1-03-0274 it has been proposed to divide each slot in a semi-static way into two parts, one for DCHs with 10 ms or larger TTI and the other for E-DCH with 2 ms TTI. Semi-static here means that it is configured by higher layers at the beginning of the connection and may be reconfigured by higher layer signaling also later. However, it should be noted that reconfiguration is a quite ‘heavy’ and slow operation which is not done frame by frame. Moreover, this division often leads to a non-optimal share of channel bits to DCH and E-DCH.